Fatty acids are important components of lipids such as phospholipids and triacylglycerols. Fatty acids containing two or more unsaturated bonds are collectively referred to as polyunsaturated fatty acids (PUFA) and are known to include arachidonic acid, dihomo-γ-linolenic acid, eicosapentaenoic acid and docosahexaenoic acid. Various physiological activities have been reported for these fatty acids (Non-patent Document 1). These polyunsaturated fatty acids are expected to have applications in various fields, but some of them cannot be synthesized in the animal body. Thus, microbial techniques have been developed for obtaining polyunsaturated fatty acids by culturing various microorganisms. Other attempts have also been made to produce polyunsaturated fatty acids in plants. In these cases, polyunsaturated fatty acids are known to be accumulated, for example, as components of lipids such as triacylglycerols within microorganism cells or plant seeds.
Such novel fatty acid synthesis in animals, plants and microorganisms is mediated by fatty acid synthetase, starting from acetyl-CoA and malonyl-CoA that is generated from acetyl-CoA by the action of acetyl-CoA carboxylase (ACC). These reactions are known to occur in the cytoplasm for animals or microorganisms and in chloroplasts for plants.
Acetyl-CoA, which serves as a source material of these fatty acids and cholesterol newly synthesized in the cytoplasm, is supplied from citrate by the action of ATP:citrate lyase (E.C. 2.3.3.8; hereinafter also referred to as ACL).
ACL is an enzyme catalyzing the following reaction.Citrate+ATP+CoA⇄acetyl-CoA+oxaloacetate+ADP+Pi  [Formula 1]
This enzyme is widely distributed in eukaryotic organisms including animals, plants and fungi, and its intracellular localization is found in the cytoplasm (Non-patent Document 2). ACL genes have been reported so far in several organisms. For example, as animal ACL genes, those derived from Homo sapiens and Rattus norvegicus have been cloned (Non-patent Document 3, Non-patent Document 4). As plant ACL genes, ACLA-1, -2, -3 and ACLB-1, -2 derived from Arabidopsis (ecotype Columbia) have been cloned (Non-patent Document 5). In the case of filamentous fungi, ACLA and ACLB genes derived from Sordaria macrospora have been cloned (Non-patent Document 6).
With respect to Mortierella alpina (hereinafter also referred to as “M. alpina”), which is a lipid-producing fungus, the cytoplasmic fraction has been reported to have ATP:citrate lyase activity (Non-patent Document 7).
Until now, these known ACL genes have been used in an attempt to increase the content of total fatty acids in hosts, for example, by highly expressing a Sordaria macrospora-derived ACL gene together with fatty acid synthetase (FAS) in yeast cells (Patent Document 1) or by highly expressing a Rattus norvegicus-derived ACL gene in plants (Non-patent Document 8).    Patent Document 1: US Patent Publication No. 2006/0051847    Non-patent Document 1: Lipids, 39, pp. 1147 (2004)    Non-patent Document 2: Adv Appl Microbiol., 51, pp. 1-51 (2002)    Non-patent Document 3: Eur J Bio Chem., 204, pp. 491-499 (1992)    Non-patent Document 4: J Bio chem., 265, pp. 1430-1435 (1990)    Non-patent Document 5: Plant Physiology., 130, pp. 740-756 (2002)    Non-patent Document 6: Curr. genet., 37, pp. 189-93 (2000)    Non-patent Document 7: Microbiology., 146, pp. 2325-2331 (2000)    Non-patent Document 8: Plant Physiology., 122, pp. 1231-1238 (2000)